The exhaust gas emitted from an internal combustion engine is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). An exhaust treatment technology in use for high levels of particulate matter reduction may include a particulate filter (“PF”) device including a filter that traps particulate matter. Regeneration is the process of removing the accumulated particulate matter from the PF device.
During regeneration, a front surface of the filter is heated to a specified temperature that allows for the trapped particulate matter to oxidize, thereby creating a flame front that generally burns longitudinally along the filter. In one approach, the exhaust gas temperature may be raised through operation of the engine during regeneration. Specifically, the exhaust gas temperature is elevated to a level sufficient to reduce the probability that the flame front may be extinguished by relatively high airflow created by accelerating the engine. However, raising the exhaust gas temperature generally requires increased amounts of fuel, which in turn will decrease fuel economy. Moreover, exhaust emissions are increased during the regeneration process.
Active regeneration refers to the process of oxidizing the accumulated diesel particulate matter in the PF device without relying on the temperature of the exhaust gas emitted by the engine, which in turn improves fuel economy. This approach may involve heating the PF device by an electrical heater until the PF device reaches the point at which the particular matter trapped in the PF device oxidizes. The electrical heater is energized if the engine is turned off, and generally receives electrical power from a vehicle battery. However, the use of an electrical heater while the engine is turned off will, over time, diminish the vehicle battery. Accordingly, it is desirable to provide an exhaust gas treatment system that provides heat needed for regeneration when the engine is turned off, while at the same time reducing or substantially eliminating battery drainage.